For measuring the weight of a target object such as a packaged product, it has been known to make use of a so-called weighing conveyor with a conveyor belt supported by a load cell serving as a weight detecting means. At one end of the weighing conveyor is a feed-in conveyor belt from which a packaged product is delivered. After its weight is measured, it is discharged onto a discharge conveyor belt and, if necessary, a selection mechanism is activated.
One of the problems associated with prior art weighing machines of this type has been that the result of measurement was severely affected by the length of the target object in the direction of its transportation (hereinafter simply referred to as its length). This problem has come to exist in the following three stages.
Firstly, a weight detecting means thus structured is directly subjected to the vibrations of the conveyor which it supports. As a result, the oscillatory load from the conveyor belt is added to the weight of the target object, and a correct weight value cannot be obtained by a direct measurement. For this reason, output signals from the weight detecting means are passed through a low pass filter for removing the components with relatively high frequencies caused by the aforementioned vibrations.
Since a low pass filter has an extremely large time constant, however, a stable output therefrom cannot be obtained until a certain length of time elapses after a target object is brought onto the weighing conveyor. In the case of a target object which is elongated in the direction of its transportation (hereinafter referred to as a long object), the entirety of the object can remain on the conveyor only for a short time, and an error is likely to occur if there is only a brief period of time available for the measurement.
In view of the problems of this type, Japanese Patent Publication Tokkai 60-79227 disclosed a weighing machine having a speed detector for the weighing conveyor such that the frequency band of the low pass filter can be adjusted according to the speed signal outputted from the speed detector. Such a weighing machine is capable of selecting an optimum frequency band for a given speed of the weighing conveyor, but there still remains the problem of measurement errors when objects of different lengths are to be weighed.
Secondly, since an error is more likely to occur in the measurement of a long object, a method has been considered whereby moment-by-moment weight data are prevented from being taken between the time when the target object reaches the weighing conveyor and the later time when the low pass filter begins to output stable weight data. In other words, a detector for the target object is provided on the object-receiving side of the weighing conveyor, and weight data are taken or not taken according to the detection signals from this detector. Such a detector usually comprises a light-emitting element and a light-receiving element placed near the weighing conveyor such that the reflected light from the target object or the screening of the light thereby may be detected. In the case of an odd-shaped target object, that is, if its top surface has protrusions and indentations, a single object may screen the light twice while passing by such a detector, and the outputted signals may indicate that two objects have passed. This will cause an error in establishing the reference time for data processing and result in incorrect measurements.
In order to overcome this difficulty, it has been known to enter from a data input means, such as a keyboard, a so-called detection inhibiting time period during which, after a detection signal is outputted from the target object detector, signals from this detector are prevented from being accepted.
If the length of the target object or the conveyor speed is varied, however, a new value of the detection inhibiting time period must be determined by measuring the length of the target object and dividing it by the belt speed, and the value thus determined must be entered through the keyboard.
Thirdly, a weighing machine of this type is adapted to keep determining an initial load value such as the weight of the conveyor, even when it is not loaded with any target object to be weighed. When there is no target object, such an initial load value is stored as the zero-point value, and the true weight of a target object is obtained by subtracting this zero-point value from the measured weight value. For this reason, it is extremely important for a weighing machine of this type to detect a no-load condition, and many methods for this purpose have been considered such as the method of monitoring the waveform of the signals from the weight detecting means in order to detect a no-load condition or that of using a timer to preliminarily set a time interval during which a no-load condition may be expected to have been established.
According to the former method, a zero-point adjustment circuit is activated after a no-load condition is detected. Thus, there is a time delay between the detection of a no-load condition and the actual start of a zero-point adjustment procedure, and this gives rise to a problem of reduced operating efficiency. By the latter method, on the other hand, the user cannot adjust to changes in the length of the target object or to the belt speed of the weighing conveyor. Thus, the timer would be set for a longer period than necessary in order to be on the safer side, and this also results in wasted time. In summary, there always remained a problem of reduced work efficiency for the weighing machine because of the waste of time between when a zero-point adjustment becomes possible and when an automatic zero-point adjustment procedure is actually started.
The present invention is for the purpose of eliminating these problems and its object is to provide a weighing machine of which the operation is not affected by changes in the length of the target object.